The goal of this project is the determination of the structures of biologically relevant, small RNAs and RNA-protein complexes by NMR. It is anticipated that the data obtained will lead to a better understanding of the chemistry that underlies RNA function in normal gene expression, and increase the likelihood that therapeutic strategies can be developed based on the inhibition of RNA function. The systems to be studied include: (1) the alpha sarcin stem/loop from eucaryotic 28S rRNA, which plays a key role in the translocation step of protein synthesis, (2) pDG07 RNA, a deletion mutant of E. coli 5S rRNA that includes the binding site for ribosomal protein L25, (3) synthetic RNA that shows hammerhead activity, and (4) synthetic RNAs containing base pairing irregularities. The ribosomal protein L25/5S RNA complex from E. coli will also be analyzed. Since experiments that depend on labelling with stable isotopes will be required to solve these problems, strategies for labelling RNAs with isotopes will be pursued, and experimental approaches will be developed that make use of isotopically labelled molecules to increase the complexity of the RNA systems accessible to study by NMR.